This invention relates to test equipment and in particular to a system for testing using noncontact electro-optical imaging of a flat panel device such as a liquid crystal display.
Diagnostic sensor placement requirements are extremely high. A flat sensor plate that is part of a sensing head which measures approximately 8 cm on each side must be placed parallel within 3 um of a flat workpiece, such as an LCD glass panel. The gap distance between the workpiece panel and sensor plate needs to be a selectable value between 7 um and 30 um and preferably between 10 um and 25 um with a tolerance of +/−0.5 um. Component hardware used to position the sensing head cannot encroach upon the clear 8 cm square aperture of the sensing head because the sensing head produces information that is read by an optical array (a CCD camera) focused on the clear aperture.
The sensing head must be able to maintain the required gap position without contacting the glass panel even when added attracting electrostatic forces resulting from a high voltage applied between the sensor plate of the sensing head and panel are present.
The sensing head must be quickly separable from the panel surface to a gap of greater than 75 um to permit translation of the elements without contact between the panel and the sensor plate as the sensing head is moved over the panel to another site. Once the sensing head arrives at the new site, the gap must be quickly reduced to the low gap position to allow the sensing head to acquire data.
Sensor placement above the panel must compensate for the variation of panel surface height from the sensor datum.